1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, and particularly to a method of fabricating a semiconductor device having “Σ” shaped embedded source/drain regions.
2. Description of the Related Art
The development of semiconductor techniques follows Moore's law, with gradually reduced critical size. A technique of embedding SiGe in recessed source/drain regions to improve channel mobility for PMOS devices has been proposed. Since SiGe has a lattice constant larger than that of Si, it can provide a compressive stress on the channel region of a PMOS device from source to drain regions, consequently enhancing hole mobility for the PMOS device.
In order to enhance the effect of stress application, a scheme of forming a “Σ” shaped recess for filling with SiGe is proposed herein.
FIG. 1 schematically illustrates a “Σ” shaped recess formed in a substrate. In this figure, a “Σ” shape is formed in the substrate 100, below the surface 130 if the substrate 100 comprising the upper portion 140 and the lower portion 150 of the recess sidewall and above the extended line 160(denoted by a dashed line) of the recess bottom 180.
The “Σ” shaped recess shown in FIG. 1, for example, can be formed through the following method.
A substrate 100 having a surface with a (001) plane orientation can be selected. As shown in FIG. 2A, first, a “U” shaped recess 210 having a bottom with the (001) plane orientation can be formed in the substrate through, for example, dry etching. The sidewalls of recess 210 then can have a (110) plane orientation.
Next, a wet etchant having orientation selectivity, such as an etchant comprising Tetramethyl ammonium hydroxide (TMAH), is utilized to etch the substrate 200 from the “U” shaped recess 210. During the etching process, the etch rate on the <100> orientation is lower than that on the <110> orientation, and the etch rate on a <111> orientation is far less than that on the <100> orientation. Thus, the “U” shaped recess 210 is etched to form a diamond-shaped recess 215, as shown in FIG. 2B. The location of the original “U” shaped recess 210 is illustrated by a dashed line. The sidewalls of recess 215 have an upper portion 240 which has substantially the (111) crystal plane orientation and a lower portion 250 which has substantially the (11 1) crystal plane orientation.
However, since the etch rate on the <100> and <110> orientations is larger than that on the <111> orientation, the flat bottom 260 of the recess 215 is liable to be etched to a cuspate shape as shown in FIG. 2B. If SiGe is epitaxially grown at the cuspate bottom of the recess 215, high-quality SiGe can not be obtained.